D4 doesn't seem to be a massive update, seems to be focused more on video, defiantly trying to compete with the 5D Mark II. The other major updates are higher-resolution, 16.2MP, full-frame CMOS sensor and the ability to shoot at 10 frames per second with autofocus. It has a massive amount of smaller tweaks, though.
Or to put it another way
SOON THE FALSE CANON GOD WILL BE CAST DOWN FROM THE GLORIOUS VIDEO THRONE, RISE MY NIKON BROTHERS, THE D4 SHALL LEAD US TO ASCENSION!